The table has four cells with white, black, dark green and pale yellow backgrounds. Each copy of the test image should be composited against the color of background that it finds itself against.
If each image is composited against the same color (for example all grey, or all black), that is a bug. If you don't see any yellow or green backgrounds, your browser does not support CSS; try the HTML 3.2 version instead.
The following images show correct and incorrect compositing: